This expression means to get to the bottom or most important part of something.

VOA: special.2010.05.16

But the moment swap returns, the moment we hit this bottom most curly brace, what conceptually happens in memory?

但是swap返回时，当我们抵达花括号底端是，在内存中发生了什么？

哈佛公开课 - 计算机科学课程节选

The root of the triad is the bottom-most pitch of that triad.

三和弦的根音是在一个三和弦最底部的音。

耶鲁公开课 - 聆听音乐课程节选